The present invention is to a process for neutralization of amine or alkanolamine heat stable salts formed when an amine solution is used in gas conditioning.
The formation of heat stable amine salts (HSAS) in amine solutions used in gas conditioning applications has long been a problem, particularly in refinery systems or when oxygen or carbon monoxide is present in the feed gas to the amine unit.
HSAS are called heat stable since they are not regenerable in the amine unit's stripping section, as are hydrogen sulfide (H.sub.2 S) and carbonate/bicarbonate/carbamate (CO.sub.2) salts. These HSAS, such as amine salts of formate, acetate, glycolate, glyoxalate, oxalate, thiocyanate, thiosulfate, sulfate, sulfite and chloride, decrease the acid gas carrying capacity of the amine and increase solution viscosity, thus increasing amine unit operating costs.
Several methods are reported to reduce HSAS in amine solutions, though the costs are very expensive and each method has practical limitations. Anion exchange is a method of removing HSAS to low levels (i.e. below 500 ppm). If cations such as iron and sodium need to be removed, then a cation anion exchange resin is also required. One disadvantage of ion exchange is the high amount of water wash required to regenerate the resin beds. In addition, ion exchange resins do not generally remove organic impurities such as hydrocarbons and triethylene glycol commonly found in amine solutions. Distillation and vacuum distillation can remove organic impurities such as triethylene glycol and hydrocarbons but the process is energy intensive since the amine and water both are carried overhead in the purification process. If HSAS are present, the amine solution must also be neutralized with strong base to prevent acids, especially formic acid and acetic acid, from going overhead into the purified amine solution.
To extend the time before having to reclaim or discard an amine solution, aqueous sodium hydroxide (caustic) or aqueous potassium carbonate are extensively used in the gas treating industry to neutralize HSAS. These bases, being stronger bases than amines generally used in gas treating operations, react with the amine HSAS to displace the amine and form the corresponding sodium or potassium salt, such as sodium or potassium salts of acetate, formate, oxalate etc. This displacement also regenerates free amine to be used for acid gas pick-up. Rooney et. al. in Hydrocarbon Processing, March 1996, pages 95 et seq. disclose the addition of chloride-free caustic (25 Baume or 19 weight percent NaOH) for neutralizing HSAS. Generally, sufficient sodium hydroxide is added to free about 80% of the amine HSAS. Limiting the freed amine to about 80% of the original HSAS is reported to provide safeguards against excess addition of sodium hydroxide, which can result in localized corrosion problems.
Although neutralization of HSAS with sodium hydroxide does extend the time before the amine solution must be reclaimed or discarded, such neutralization still results in the formation of a number of solids in the amine solution. These solids are detrimental to the operation of the amine solution and lead to amine solution losses, increased maintenance activities and occasionally plant shutdowns. It would therefore be advantageous to have a process for neutralization of amine solutions containing HSAS whereby the amount of solids formed in the amine solution is reduced and/or the useful life of the amine solution is increased.